1995 Pontiac Grand Am Really bad Gas Mileage

Hello. I have a 1995 grand am with a quad 4. I replaced the plugs with bosch quadrupile platinum, new fram air filter, new plug boots, halvoline synthetic blend oil, new filter and some seafoam in the tank. I am getting 17 mpg at the best. I am very frustrated. I also have new tires all the way around. The MIL light does not come on, nor does any other light. I have ran diagnostics and everything is dandy. What could be causing this. I baby this car, and no one else drives it. I also get good gas. I think it might be the O2 sensor, but im not sure. Please help. Thanks! Please copy your answer and email me, if you are able to woitel3000@mchsi. Com. Anyone is welcome to respond